. The theory of evolution in the light of facts. here are, however, true Turbellaria is shown by theirdifferent embryonic development from that of theTapeworms. There would therefore also have been in the Tapeworms a markedalteration in the genesis ofthe embryo ; the adult formsof a Tapeworm and of suchparasitic Turbellaria would bythemselves form no reason forestablishing different classes. As between Tapeworms andTurbellaria so is the separationbetween Turbellaria and Dis-toma (sucking worms) notalways to be clearly are free-living Turbellariawith suction apparatus but nohooks
. The theory of evolution in the light of facts. here are, however, true Turbellaria is shown by theirdifferent embryonic development from that of theTapeworms. There would therefore also have been in the Tapeworms a markedalteration in the genesis ofthe embryo ; the adult formsof a Tapeworm and of suchparasitic Turbellaria would bythemselves form no reason forestablishing different classes. As between Tapeworms andTurbellaria so is the separationbetween Turbellaria and Dis-toma (sucking worms) notalways to be clearly are free-living Turbellariawith suction apparatus but nohooks, which are only service-able to parasites. Such Tur-bellaria could easily becomeParasites (Ekto-parasites) ifthey could thereby obtainnourishment more easily. Wehave namely supporting evidenceby observation which shows howopportunity alone may renderan animal a facultative parasite whether the oppor-tunity be artificially or spontaneously provided. ^ Thus can animals, which live normally in theexcreta of man, also develop themselves inside the. Fig. 32.—Planaeia. Plan of a free-living , eye ; ci, cilia ; D, frontmain intestine; da, branchof same ; Dl, left rear, andDr, right rear main intestine ; g, brain ; ge, germinal gland ; m, mouthph, amis ; T, feelers ; tetesticle bladder ; u, ovariumvi, yolk gland. {After von Gra^.) SUGGESTIONS FOR RELIABLE HYPOTHESES 185 -Dl -VI human intestine if by chance their eggs find their way there —as, for instance, the larvae of the flies Eristalis tenax and Anthomyia canicularis} The worm Leftodera afpendiculata lives mostly in rotting material in the soil; if it finds entry into the hiding-place of a snail in close vicinity (which thus shares its habitat) it can also flourish there very well. Certainly by such examplesnothing is shown to demonstratehow a parasite by opportunityonly may become a true one whichcan only exist in a definite foreignorganism. We can, in the firstplace, only say that frequent asso-ciation at close qu
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